This invention relates to aircraft distance measuring equipment (DME), and more particularly to improvements in digital DME systems.
Conventionally, DME operation involves the transmission of a radio signal of specified character from an aircraft to a ground station. After specified delays and under known constraints at the ground station, a reply signal is radioed back to the aircraft. Once the return signal is identified, and known time delays are accounted for, the duration between transmission and reception is decoded to yield distance from the ground station to the aircraft.
Due to the nature of the data utilized to compute aircraft to ground station distance, other parameters of the flight of the aircraft also may be developed. DME systems are available which also provide the pilot with an indication of aircraft ground speed and of the time which will be required for the aircraft, travelling at the present speed, to reach the ground station. In U.S. patent application Ser. No. 571,845 of R. L. Powell, et al., filed Apr. 28, 1975 and assigned to the assignee hereof, there is described a method and apparatus for developing a ground speed indication for digital DME systems. In that system, a voltage controlled oscillator provides a pulse signal which, when frequency scaled, is representative of aircraft velocity. Pulses representative of passage of the aircraft over distance intervals are coupled to one input of a comparator-integrator, and the VCO output pulses are further divided and coupled to the other input terminal of the comparator-integrator. The integration voltage controls the VCO.
For the ground speed calculation of the referenced copending application, and for other DME system functions as well, operation is dependent on provision of signals representing passage of the aircraft over distance intervals. Conventional DME systems operate to a degree of precision of tenths of nautical miles, so it is useful to provide signals representing passage of the aircraft through such intervals. For reasons inherent in the design of conventional DME systems, however, provision of totally accurate distance passage signals is neither simple nor straight forward.
For example, each ground station simultaneously serves many aircraft, and due to operational constraints, may be forced at times to serve some aircraft and ignore others. Also, atmospheric or the like disorders may prevent a ground station response from being properly decoded at an aircraft. Ground station and aircraft antenna "shadow periods" may also impede DME ranging. Hence, unless these potential difficulties are accounted for, the DME may fail to discriminate significant changes of aircraft to ground station distance.
It is accordingly a primary object of the present invention to provide distance decoding logic whereby distance change is accurately noted, even though considerable time may lapse between successive accurate DME ranging calculations.
It is a further object to provide ancillary logic whereby reversals of aircraft direction are accurately noted. That is, noise conditions or the like may temporarily appear to be distance reversals, and it is an object of the present invention to discriminate such "false reversals" from true aircraft directional change, and to maintain the distance decoding appropriately.
Further, it will be understood that satisfaction of the above objects in DME systems will also provide a apparatus which finds utility in other systems based on similar data processing capability. That is, many systems require continuous monitoring of coded data change, and advantages accrue in such systems if the degree of change which has occurred may be effectively determined in a manner as set forth herein for DME systems. It is accordingly another object of the present invention to provide data change detection apparatus and methods which are applicable to a wide variety of systems which have data processing requirements similar to those of DME systems.